Color component aperture stops in projection display system

ABSTRACT

A projection display system employs one or more color modifying aperture stops, such as apodizing aperture stops, to provide high contrast, balanced color and high throughput. One projection system includes a reflective liquid crystal-on-silicon light valve positioned with a polarizing beam splitter, such as a wire grid polarizing beam splitter, for each of the primary color component light paths to separately impart image information into each of the primary color components of light. A color combiner receives and combines the primary color components of light with imparted image information to provide light representing a polychromatic display image. At least one aperture stop is positioned along at least one of the primary color component light paths to balance relative intensities of the primary color components of light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Application Ser. No. 10/337,474filed Jan. 7, 2003 which claims priority to Provisional Application No.60/347,103 filed on Jan. 7, 2002, the entire disclosure of ApplicationSer. No. 10/337,474 is considered as being part of the disclosure of thecurrent application and is hereby incorporated by reference herein.

The present invention relates to electronic projector optical systemsand, in particular, to employing shaped aperture stops to improve colorbalance and image contrast.

BACKGROUND AND SUMMARY OF THE INVENTION

Various arrangements of optical layouts are known for projection systemwith reflective liquid crystal displays. Examples are described in U.S.Pat. Nos. 6,309,071 and 6,113,239 and in High Contrast Color SplittingArchitecture Using Color Polarization Filters, Michael G. Robinson etal, SID 00 Digest, p. 92–95. One optical layout described in theRobinson et al. article employs proprietary polarization filtertechnology (i.e., ColorSelect™ polarization filter technology),available from ColorLink, Inc. of Boulder, Colo, USA, to achieve areported contrast of more than 500:1.

However, contrast in these known systems is limited due to use ofMacNeille prisms as the polarization beam splitters (PBSs) in differentarrangements. A MacNeille prism PBS has limited contrast due to skew-raydepolarization effects, as described in U.S. Pat. No. 5,327,270. Thedepolarized light reduces the contrast of reflective electronicprojection displays, and particularly those employing liquidcrystal-on-silicon (LCOS) light valves. As described in the '270 patent,compensation for the skew-ray depolarization requires an additionalquarter-wave plate, which increases cost, requires precision alignmentand restricts the range of operating temperatures.

Generally, reflective liquid crystal on silicon (LCOS) light valves haveseveral advantages for use in projection displays, including small pixelsize, high aperture ratio, and fast response. As the numerical aperture(NA) of a system using reflective light valves is increased to maximizeimage brightness, however, contrast decreases. This reduction incontrast is largely due to the interaction between the non-idealretardance of the light valves and compound angle depolarization by thetilted polarizing beamsplitters (PBSs) typically used in such systems;the contrast varying approximately with the inverse square of thenumerical aperture. In addition to reduced contrast, increased NAresults in poorer image quality due to increased geometric aberration inthe projection lens.

Another limitation of conventional systems is the color temperature orbalance of the light. Projection systems typically require a lamp withlong lifetime and extremely small source of light, such as is providedby high-pressure mercury lamps (e.g., UHP type, available from PhilipsElectronics). These lamps produce a discontinuous spectrum and arerelatively deficient in one or two primary colors, requiring at leastone of the primaries (typically green, and sometimes green and blue) beattenuated to obtain an acceptable white point. This is typically doneby limiting that primary to a narrower bandwidth than required to obtaina satisfactory color gamut. For example, the color separation filtersare modified to reduce the spectral width of the green and blueprimaries, causing them to become more saturated than those specified inthe SMPTE broadcast standard and restricting their dynamic range.

Accordingly, the present invention provides high contrast, balancedcolor and high throughput in a wide variety of electronic projectors,such as a multi-path, reflective liquid crystal-on-silicon (LCOS)projection display system.

In one implementation, a reflective liquid crystal-on-silicon projectionsystem includes an illumination system that generates polychromaticlight. A color separation system, such as a cross-dichroic, ispositioned to receive the polychromatic light and to separate it intoprimary color components of light that are directed along separateprimary color component light paths. At least one color modifying (e.g.,balancing) aperture stop is positioned along at least one of the primarycolor component light paths to balance relative intensities of theprimary color components of light.

A reflective liquid crystal-on-silicon light valve is positioned with apolarizing beam splitter, such as a wire grid polarizing beam splitter,for each of the primary color component light paths to separately impartimage information into each of the primary color components of light. Acolor combiner receives and combines the primary color components oflight with imparted image information to provide light representing apolychromatic display image.

In another implementation, a color balancing aperture stop such as anapodizing aperture stop may be positioned to color balance the lightbefore it is color separated. For example, the apodizing aperture stopmay include an annular color filter corresponding to the primary colorcomponent of light of the primary color component light path in whichthe apodizing aperture stop is positioned.

The one or more aperture stops provide attenuation by reducing thenumerical aperture (or increasing the F-number) of one or more primarycolor channels. The aperture stops may be implemented in various ways,including use of a smaller illumination system aperture stop, where aseparate aperture stop location exists for each primary, or use of asmaller projection lens aperture stop, where separate projection lensesare used for each primary, or use of an annular color filter at theaperture stop of the illumination system or projection lens, where acommon illumination system or projection lens is used for all primaries.

Additional description and implementations of the present invention willbe apparent from the detailed description of the preferred embodimentthereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an embodiment of a reflective projection displaysystem according to the present invention.

FIGS. 2A–2F are diagrams illustrating various alternative shapedaperture stops according to the present invention.

FIG. 3 is a top view of one implementation of an illuminationcross-dichroic.

FIG. 4 is a top view of one implementation of imaging cross-dichroic.

FIG. 5 is a simplified illustration of the operation of a wire gridpolarizing beam splitter.

FIG. 6 is a diagram of a pair of raytracings illustrating optical rayspassing through relay optics of a projector of the present invention.

FIG. 7 is a simplified diagram of an electronic (LCD) projector incombination with a color selective apodizing aperture stop according tothe present invention.

FIG. 8 is a front view of an apodizing aperture stop with a circularinner edge configuration.

FIG. 9 is a front view of an apodizing aperture stop with a roundedcruciform inner edge configuration.

FIG. 10 is a front view of a stacked apodized aperture stop that may beused as an illumination aperture stop position.

FIG. 11 is a front view of another example of a stacked apodizedaperture stop.

FIG. 12 is a simplified diagram of an electronic (LCD) projector with 90degree twisted nematic LCDs in combination with a color selectiveapodizing aperture stop according to the present invention.

FIG. 13 is a front view of an apodizing aperture stop adapted toasymmetric contrasts of multiple twisted nematic LCDs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a diagram of an embodiment of a reflective projection displaysystem 10 illustrating an example of an operating environment for thepresent invention. Projection display system 10 (sometimes referred toherein as projector 10) includes three color component optical paths 12(only one shown) that correspond to the respective primary color lightcomponents red, green and blue. For purposes of clarity, FIG. 1 showsonly one of the color component optical paths, which is designated colorcomponent optical path 12G to correspond to the green primary colorcomponent.

It will be appreciated that the red and blue color component opticalpaths will be the same as, but in places offset from, green colorcomponent optical path 12G. Elements of projector 10 that are specificto one primary color light component will be indicated by acorresponding alphabetic suffix (i.e., “R,” “G,” or “B”). Elements ofprojector 10 that are not specific to one primary color light componentwill not include such a suffix. Accordingly, the following descriptionwill be directed to the elements associated with the green colorcomponent, but will be similarly applicable to the elements associatedwith the red and blue color components.

An illumination system 14 having an elliptical reflector 16 and a widespectrum (e.g., “white”) light source 18 directs illumination light 20through a light pipe integrator 22 (hollow or solid) and relay optics 24to an illumination cross-dichroic 26.

Referring to FIG. 1, green optical path 12G passes from crossed-dichroic26 and is directed by a fold mirror 38G through relay optics 40G and acolor balancing aperture stop 42G according to the present invention. Apolarizing beam splitter (PBS) 48G and a reflective light valve 50G,such as a liquid crystal-on-silicon (LCOS) LCOS light valve 50G, receivethe light from color balancing aperture stop 42G. In one embodiment,polarizing beam splitter 48G includes a wire grid polarizer 52G, such asa ProFlux™ polarizer available from Moxtek, Inc. of Orem, Utah, USA.Such a wire grid polarizer 52G is described in U.S. Pat. No. 6,122,103.As implemented with wire grid polarizer 52G, PBS 48G may be referred toas a wire grid PBS 48G.

After being modulated by LCOS light valve 50G, the modulated green colorcomponent is reflected by LCOS light valve 50G back to wire gridpolarizer 52G, which reflects the modulated green color component to animaging cross-dichroic 54. Imaging cross-dichroic 54 is identical toillumination cross-dichroic 26 and functions to combine the red, green,and blue modulated color components and pass them to a projection lensassembly 56, which projects the full-color display image onto a displayscreen (not shown). Projector 10 may be operated in either a frontprojection format or a rear projection format.

In operation, light from illumination system 14 passes throughintegrator 22, which creates a uniform intensity illuminationdistribution at an integrator exit window 60. Illuminationcross-dichroic 26 splits the illumination light into three colors (red,green, and blue), which are directed to three separate LCOS light valves(only LCOS light valve 50G shown) by three identical sets of relayoptics (only relay optics 40G shown) and three identical fold mirrors38R, 38G, and 38B (FIG. 3). Relay optics 24 and color component relayoptics (only 40G shown) create images of integrator exit window 60 at anoptically active area of each LCOS light valve.

Color balancing aperture stop 42G functions to attenuate a primary colorcomponent (e.g., green) to obtain an acceptable white point. Theattenuation is achieved by reducing the numerical aperture (NA) of thatprimary. The numerical aperture may be reduced in a variety of ways,including a smaller illumination system aperture stop, where a separateaperture stop location exists for each primary. In anotherimplementation, a smaller projection lens aperture stop may be used,where separate projection lenses are used for each primary.

It will be appreciated that some projection systems that have a commonillumination system or projection lens that is used for at least twoprimaries, rather than separate color component elements as inprojection system 10. In these projection systems, an annular colorfilter at the aperture stop of the common illumination system orprojection lens may be used to provide color balancing, as describedbelow in greater detail.

This reduction in numerical aperture increases the contrast and imagequality of that primary. Particularly in systems requiring attenuationof green light, such as those using high-pressure mercury lamps, thehigher contrast and image quality in one primary significantly increasesthe visual perception of overall contrast and image quality.

FIGS. 2A–2F are diagrams illustrating various alternative shaped colorbalancing aperture stops 120A–120E according to the present inventionthat may be used as color balancing aperture stop 42G and any othercolor balancing aperture stop in projection system 10. (Aperture stops120A–120E are sometimes referred to collectively as aperture stops 120.)Exemplary dimensions are indicated for shaped aperture stops 120 forpurposes of illustrating exemplary proportional dimensions. It will beappreciated that the shaped aperture stops 120 could be formed withdimensions other than those indicated. As a basis for describing shapedaperture stops 120, a conventional aperture stop 126 is illustrated withan opaque face 128 and an exemplary circular aperture 130 with anexemplary diameter of 62.5 mm.

Shaped aperture stop 120A includes an opaque face 122A and a generallyoval or elliptical aperture 124A. Shaped aperture stop 120B includes anopaque face 122B and a large cropped circular aperture 124B. Largecropped circular aperture 124B includes large opposed circular segments132B that are positioned between a pair of straight-cropped sides 134B.Circular segments 132B are large in that they encompass larger circularsegments of aperture 124B than do cropped sides 134B.

Shaped aperture stop 120C includes an opaque face 122C and a smallcropped circular aperture 124C. Small cropped circular aperture 124Cincludes small opposed circular segments 132C positioned between a pairof straight-cropped sides 134C. Circular segments 132C are small in thatthey encompass smaller circular segments of aperture 124C than docropped sides 134C.

Shaped aperture stop 120D includes an opaque face 122D and analternative cruciform aperture 124D. Cruciform aperture 124D includessmall opposed circular segments 132D positioned between a pair ofstraight-cropped sides 134D and transverse extensions 136D. Shapedaperture stop 120E includes an opaque face 122D and a small circularaperture 124E.

Table 1 lists optical throughput for each of shaped aperture stops 120,both in measured lumens and as a percentage of the throughput ofconventional aperture stop 126 for a given light source (e.g., a 20 mmimage at F/2.2).

TABLE 1 Lumens % through Standard 3100 100%  Elliptical (120A) 3060 99%Big cropped (120B) 2950 95% Small cropped (120C) 2500 81% Cross (120D)2330 75% Small Circle (120E)  920 30%Table 1 illustrates the range of throughput variations that can beachieved with the various alternative shaped aperture stops 120. Inaddition, shaped aperture stops 120A–120D have elongated aspect ratiosthat to varying degrees preferentially block light at the extremecorners of the light bundle. Light at these corners is typically mostsubject to off-axis or skew rays (i.e. contrast or performance degradinglight) or stray paths, so blocking light at these corners can provide agreater improvement in contrast.

Aperture stop 42G sets a working F-number, or numerical aperture, forthe corresponding color component light path (e.g., green). Each colorcomponent light path includes a corresponding aperture stop. By usingapertures with appropriate diameters the amounts of red, green and bluelight can be controlled to provide a desired color temperature on thescreen while increasing contrast.

An exemplary implementation of projection system 10 employs as lightsource 18 a high-pressure mercury lamp (UHP type), which has limitedintensity in red. To obtain a desired color temperature (colorcoordinates for the white screen), the amount of green light is reducedby about 35%. With aperture stop 42R having a base F-number of 2.8,aperture stop 42G may be formed with an F-number of 3.5 to effect a 35%reduction in green light that will significantly increase the overallcontrast of the panel. Hence, stop 42G functions to improve the contrastand image quality of projector 10 while preserving image or displaybrightness.

Some conventional color light-valve projection display systems use highintensity discharge (HID) light sources that produce a discontinuousspectrum and are relatively deficient in one or two primary colors.These systems require at least one of the primaries, typically green, tobe attenuated to obtain an acceptable spectral balance or “white point.”Typically, attenuation of such a primary color component is achieved bylimiting that primary to a relatively narrow bandwidth to obtain asatisfactory color gamut.

For example, a rear-projection television system may use a high pressuremercury discharge lamp referred to as the “Ultra-High Performance” (UHP)lamp, available from Philips Electronics. The UHP lamp is relativelydeficient in red and requires considerable attenuation of green and bluelight to achieve an acceptable white point. This is typically done bymodifying the color separation filters to reduce the spectral width ofthe green and blue primaries, causing them to become more saturated thanthose specified in the SMPTE broadcast standard and restricting theirdynamic range.

FIG. 3 is a top view of one implementation of illuminationcross-dichroic 26, which includes crossed dichroic coatings 142 and 144positioned between inclined faces of a set of prisms 146, as is known inthe art. Dichroic coatings 142 and 144 reflect and transmit differentcolor components so that illumination light 20 received at an incidentface 148 is separated into the color components reds, green and blue anddirected out respective exit faces 150R, 150G, and 150B.

FIG. 4 is a top view of one implementation of imaging cross-dichroic 54,which is the same as illumination cross-dichroic 26 and includes crosseddichroic coatings 164 and 166 positioned between inclined faces of a setof prisms 168. FIG. 4 illustrates positioning of wire grid PBSs 48R,48G, and 48B adjacent incident faces 170R, 170B, and 170G, respectively.FIGS. 3 and 4 illustrate the corresponding positioning of identicaloptical elements for each of the three color components symmetricallyabout a projector centerline 172 (FIG. 1). It will be appreciated that acold mirror could be inserted between elements of relay optics 24 toremove excess heat from projection system 10. Also, one or moreadditional folds can be arranged between the elements of relay optics 40to provide alternative or better packaging.

Illumination and imaging cross-dichroics 26 and 54 are substantiallyidentical. In one implementation cross-dichroics 26 and 54 are of anSPS-type in which dichroic coatings 142, 144, 164, and 166 reflectS-polarized light and transmit P-polarized light of selected colors. Forexample, coatings 142 and 164 may reflect S-polarized red light, andcoatings 144 and 166 may reflect S-polarized blue light, alltransmitting P-polarized green light.

Cross-dichroics 26 and 54 each include three half-wave plates, one foreach color component (not shown), as is known in the art, to correlatethe polarization of light through cross-dichroics 26 and 54 with thepolarizations for PBSs 48R, 48G, and 48B. S- and P-polarizations areconventional nomenclature referring to a pair of orthogonal linearpolarization states in which, with regard to a polarization selectivedielectric film, S-polarized light can be said to “glance” off the filmand P-polarized light can be said to “pierce” the film.

Accordingly, projection system 10 uses one polarization for the greenchannel and an orthogonal polarization for the red and blue channels.These polarizations allow use of overlapping spectrum for the blue andgreen channels to increase the system throughput. Overlapping spectrumbetween the red and green channels cannot be used due to colorimetryconsiderations.

In some implementations, the desired display color characteristicscannot be achieved from the color properties of cross-dichroics 26 and54 alone. Additional color correction dichroic filters (one long-pass,and two short-pass—not shown) may be used to provide color purity. Theselow-cost correction filters can be inserted practically anywhere in theillumination path and might preferably be added at an illumination stop(i.e. combined function with color selective apodizing filter layer(s)).

FIG. 5 is a simplified illustration of the operation of PBS 48G, forexample. The operation of PBSs 48R and 48B would be analogous, butmodified for the different polarization states of the red and blue colorcomponents,

P-polarized green illumination light passes through a wire gridpolarizer 52G, which is oriented to pass light with the P-polarizationstate of the green color component. The light strikes LCOS light valve50G and is modulated according to a green color component display imageand reflected as S-polarized modulated light back toward wire gridpolarizer 52G. An clean-up polarizer 176G is positioned at an entranceface 174G of cross-dichroic 54 and can be a low-cost, off-the-shelf filmpolarizer. Measured system contrast of the optical arrangement of FIG. 5exceeds 3200:1. This system contrast was measured with a front-surfacemirror and quarter-wave plate combination substituted for LCOS lightvalve 50G to separate contrast of the optical arrangement from thecontrast of the LCOS light valve 50G itself.

This optical arrangement of FIG. 5 does not suffer from skew-raydepolarization (so that no compensating quarter-wave plate is required),has a very high polarization extinguish ratio, works within a largetemperature range, and can withstand a high light intensity. Wire gridpolarizer 52G can be made on a flat glass substrate and can be used inthe reflection mode in the imaging optical path as shown in FIG. 1, forexample. The flatness of PBS 48G does not create significant deformationof the wavefront and provides high image quality for projector 10.Unfortunately this flat PBS 48G is too thick to be used in a transmittedmode in the imaging path: astigmatism created by this tiltedplano-parallel plate is too great. This flat PBS 48G can be accepted inthe illumination path of a transmitted mode.

To obtain uniform color distribution across a projection display screen(i.e., to avoid ‘no color shift’), the cross-dichroics 26 and 54 shouldbe placed in the telecentric space of the system. To obtain uniformdistribution of the light on the white screen, the telecentricity shouldbe provided in the space of integrator exit window 60. Accordingly,relay optics 26 and 40 should be telecentric in three spaces: as tointegrator exit window 60, as to illumination cross-dichroic 26 and asto imaging cross-dichroic 54. With no active optical componentspositioned between them, imaging cross-dichroic 54 and each wire gridPBS 48 also work in the telecentric space, which also supports uniformcontrast across the image on the screen.

FIG. 6 is a diagram of a pair of raytracings 180 and 182 illustratingoptical rays passing through relay optics 24 and 40. Raytracing 180corresponds to projection system 10 as viewed in FIG. 1. Raytracing 182corresponds to projection system 10 as viewed from a direction 184 (FIG.1). Raytracings 180 and 182 illustrate three regions in which relayoptics 24 and 40 of projection system 10 are formed to providetelecentricity, thereby providing no color shift, uniform distributionat the white and at the dark screen across the image. As a result,projection system 10, and in particular relay optics 24 and 40, may besaid to be triply telecentric.

In particular, relay optics 24 (i.e., the “target space”) are formed tobe telecentric to utilize and maintain brightness uniformity across thefield of view. Light pipe integrator 22 provides the same angulardistribution of brightness from point to point across integrator exitwindow 60. To provide uniform illumination across the screen thecollection efficiency across pipe exit window 60 should be the same. Toutilize this illumination uniformity, relay optics 24 are formed to havean entrance pupil at infinity (i.e., a telecentric entrance pupil).

Telecentricity is also maintained in the region between relay optics 24and 40, which includes illumination cross-dichroics 26. Generally, thespectrum properties of dichroics, such as those included incross-dichroic 26, strongly depend on angle of incidence. To avoiddeviations of spectrum across the image, a phenomenon called colorshift, the light in this region between relay optics 24 and 40 isprovided with an identical angular structure for all points of field ofview. Such an identical angular structure is another way of referring totelecentricity.

Finally, in the image space of relay optics 40G and light valve 50telecentricity provides uniform contrast across the field of view. Inother words, all points of light valves 50 are in the same conditionswith respect to incoming light and, assuming good reflection surfaceinside the LCOS device, with respect to the outgoing light as well. Withno active optical elements positioned between light valve 50 and imagingcross-dichroic 54, the telecentricity in this space also provides a nocolor shift condition for imaging cross-dichroic 54.

It will be appreciated that projector 10 employing LCOS light valves 50is but one example of an electronic projection display system that canemploy color balancing aperture stops in accordance with the presentinvention.

FIG. 7 is a simplified diagram of an electronic (LCD) projector 200 incombination with a color selective, color balancing aperture stop 202according to the present invention. Electronic projector 200 receivespolychromatic light 204 via color selective, color balancing aperturestop 202.

Electronic projector 200 includes a pair of color selective mirrors 204and 206 that separate the polychromatic light 204 into color components(e.g., red, green, and blue) that are directed through respectivetransmissive LCD/polarizer stacks 208R, 208G, and 208B, which impartimage display information into the light. A conventional X-cube 210combines the color components with image display information and directsthe combined light to a projection lens 212.

FIGS. 8 and 9 are front views of respective exemplary color selective,color balancing aperture stops 220 and 222 that may be used as aperturestop 202.

With reference to FIG. 8, aperture stop 220 includes an opaque outerannulus 224 with a circular inner edge 226. Light striking outer annulus224 is blocked in the conventional manner of an aperture stop. Acircular colored filter annulus 228 is positioned inside inner edge 226and transmits one or two selected color bands. A central circularoptical aperture 230 allows all colors of light to pass withoutfiltering and may be a physical aperture or a transparent substrate.With the different transmissivities of circular colored filter annulus228 and circular optical aperture 230 aperture stop 220 may be referredto as an apodizing aperture stop 220.

In one implementation, for example, the green color component of theillumination light is to be reduced relative to the red and bluecomponents to improve the color balance. In this implementation, coloredfilter annulus 228 may be formed of a magenta color filter that passesred and blue light components. The red and blue light components have anaperture defined by inner edge 226, and the green component has anaperture defined by optical aperture 230. As a result, apodizingaperture stops 220 improves the color balance by selectively reducingthe green component relative to the red and blue components.

With reference to FIG. 9, aperture stop 222 can provide greaterimprovement in image contrast if reduction of aberration effects can beless than optimal. Aperture stop 222 includes an opaque outer annulus234 with an inner edge 236 having, for example, a circular shape. Lightstriking outer annulus 234 is blocked in the conventional manner of anaperture stop. A colored filter annulus 238 is positioned inside inneredge 236 and transmits one or two selected color bands corresponding tothe primary color component directed through the stop (e.g., magenta).Aperture 239 can have other shapes, such as those shown in FIG. 2.

A central rounded cruciform optical aperture 239 allows light to passwithout filtering and may be a physical aperture or a transparentsubstrate. For example, rounded cruciform optical aperture 239 and thecolor selective stop or color filter annulus 238 can be oriented toeliminate rays which would otherwise be incident at large compoundangles, while passing rays at equal non-compound angles. With thedifferent transmissivities of colored filter annulus 238 and cruciformoptical aperture 239, aperture stop 222 may also be referred to as anapodizing aperture stop.

Aperture stops 220 and 222 function to limit, restrict, or otherwiseshape the light cone (i.e., cone of illuminating light) to improvesystem contrast. Accordingly, the improved contrast provided by aperturestops 220 and 222 is in contradistinction to reduced contrast inconventional systems provided by relatively increased numericalapertures.

In one implementation, aperture stop 220 may be applied to narrow theintensity and angular extent of the green light relative to the red andblue components. Green light can account for up to about 80% of the RGBbalance and so can predominantly contribute to the contrast of thesystem. Improves green contrast provided by an aperture stop 220 canimprove overall system contrast.

It will be appreciated that apodized aperture stops 220 and 222 may bepositioned at any other optical position in an electronic projector orprojection display system, whether or not the optical positionconventionally would have an aperture stop. For example, either ofapodized aperture stops 220 and 222 may be positioned at a position 62(FIG. 1) in the relay optics 24 of projection system 10 (FIG. 1) as asubstitute for aperture stop 42G.

In the alternative aperture position 62 (FIG. 1), with the colorcomponents of the illumination light are not yet separated, apodizedaperture stops 200 and 202 may include a color filter element forpreferentially blocking one color component (e.g., green) while theother color components are passed (e.g., red and blue, or magenta). Inyet other embodiments, apodized aperture stops 200 and 222 may includestacked color filter elements for preferentially blocking two colorcomponents (e.g., green and blue) by different amounts relative to theremaining color component (e.g., red).

FIG. 10 is a front view of a stacked apodized aperture stop 240 that maybe used at an aperture stop position 62, for example, to preferentiallyblock two color components (e.g., green and to a lesser degree blue) bydifferent amounts relative to the remaining color component (e.g., red).Stacked aperture stop 240 includes a large diameter red filter annulus242 that preferentially passes red light, an intermediate magenta filterannulus 244 that preferentially passes red and blue light, and a centraloptical aperture 246 that allows light to pass without filtering.Central optical aperture 246 may be a physical aperture or a transparentsubstrate and may be circular, as shown, or any other shape as describedherein.

It will be appreciated that stacked apodized aperture stop 240 may beformed in a wide variety of color filter arrangements according to thecolor components to be proportionally reduced or increased. For example,stacked apodized aperture stop 240 could alternatively be formed withred filter annulus 242 omitted and magenta filter 244 extending over theannular region otherwise covered by red filter annulus 242. In addition,it will be appreciated that aperture stop 240 could further include anopaque outer annulus (not shown) of a diameter greater than magentafilter annulus 244 so that all color components, including red, areblocked at an outer extent.

As another example, FIG. 11 is a front view of another of a stackedapodized aperture stop 250. An opaque face 252 has a generally circularaperture 254 within which an annular magenta (i.e., red and blue) filter256 is positioned. Annular magenta filter 256 includes an inner aperture258 that has no color filtering. In this illustration, clear inneraperture 258 has an elliptical shape. Color selective apodizing aperturestop 250 functions to reduce the proportion of green light relative tothe red and blue light.

Apodized aperture stops 200, 202, and 240, and 250 may be consideredembodiments of shaped aperture stops that are shaped or sized relativeto each other so as to adjust the balance of color components in aprojection display system, such as projection system 10 or projectionsystem 200. It will be appreciated, however, the apodizing of aperturestops 200, 202, and 240, and 250 with annular color filters are onemanner of adjusting color component balance. Shaped aperture stopsaccording to the present invention may alternatively be formed withoutthe apodizing color filters of aperture stops 200, 202, and 240, and250. In addition, it will be appreciated that apodizing color filters ofaperture stops 200, 202, and 240, and 250 may be of different shapes,including having outer edges that are not circular.

FIG. 12 is a simplified diagram of an electronic (LCD) projector 260 incombination with a color selective, color balancing aperture stop 262according to the present invention. Electronic projector 260 receivespolychromatic light 264 via color selective, color balancing aperturestop 262. Color selective, color balancing aperture stop 262 is shown ingreater detail in FIG. 13.

Electronic projector 260 includes a pair of color selective mirrors 266and 268 that separate the polychromatic light 264 into color components(e.g., red, green, and blue) that are directed through respectivetransmissive LCD/polarizer stacks 270R, 270G, and 270B, which impartimage display information into the light. A conventional X-cube 272combines the color components with image display information and directsthe combined light to a projection lens 274.

LCD/polarizer stacks 270R, 270G, and 270B include respective 90 degreetwisted nematic LCDs 276R, 276G, and 276B, which characteristically haveasymmetric contrasts at different viewing angles. As is known in theart, such asymmetric contrasts are commonly represented in a polarcontrast plot or graph. In this implementation, display contrast can beincreased by blocking angular components 278R, 278G, and 278B from therespective red, green, and blue color components of light.

It will be appreciated that the angular components 278R, 278G, and 278Bare positioned throughout the respective color component light bundles.Blocking angular components 278R, 278G, and 278B provides increasedcontrast because the corresponding LCD has poorer performance for thatangle space. The same direction respective to each of LCDs 276R, 276G,and 276B is blocked, assuming that all devices are made with same liquidcrystal, rubbing angles, etc. Due to the nature of the optical layout,278R does not appear to come from the same corner as 278G and hence theunusual color-selective aperturing used to restrict these differentcolored bundles so as to limit contrast-reducing light appropriately foreach color channel. Aperture stop 262 blocks regions that correspond toangular components 278R, 278G, and 278B at respective LCDs 276R, 276G,and 276B.

Color selective, color balancing aperture stop 262 includes a cyan colorselective filter 280C, a magenta color selective filter 280M, and ayellow color selective filter 280Y positioned in adjacent corners. Anopen aperture region 282 passes light of all colors. Cyan filter 280Cfunctions to block red light, magenta filter 280M functions to blockgreen light, and yellow filter 280Y functions to block blue light. Thecorners in which filters 280C, 280M, and 280Y are positioned correspondto the angular components 278R, 278G, and 278B (FIG. 12). Colorselective, color balancing aperture stop 262 is adapted to theasymmetric polar contrasts of LCDs 276R, 276G, and 276B to provideincreased contrast to electronic projector 260.

As an alternative implementation, projection system 10 may be formedwith conventional polarizing beam splitters that employ prism structuresas alternatives to wire grid PBSs 48R, 48G, and 48B. However, suchconventional PBSs can be especially sensitive to ‘skew rays’ that reducecontrast in the system due to geometrical rotation of the polarizingaxis (i.e., depolarization). In such an implementation, aperture stop 82can reduce such skew rays by narrowing the illumination in one axis, orin two axes by clipping out the corners of a normally circular pupil.

As another implementation, the illumination light cone can be narrowedin one axis by employing a light pipe integrator 22 that is tapered inthe one axis. For example, in a high definition TV (HDTV) implementationproviding images with a rectangular 16:9 aspect ratio, a one-axistapered light pipe integrator 22 could have a square entrance face toadvantageously gather the light from an elliptical reflector. To providean exit face with an appropriate 16:9 aspect ratio, the one-axis taperedlight pipe integrator 22 would function to ‘squeeze’ the light bundleinto a smaller angle. This could allow, for example, an F/2.5 verticalextent with an F/4.0 horizontal extent.

In view of the many possible embodiments to which the principles of ourinvention may be applied, it should be recognized that the detailedembodiments are illustrative only and should not be taken as limitingthe scope of our invention. Rather, we claim as our invention all suchembodiments as may come within the scope and spirit of the followingclaims and equivalents thereto.

1. An electronic projection system, comprising: an illumination systemgenerating polychromatic light with plural primary color components; atleast one color modifying aperture stop to selectively block at leastone of the primary color components of light relative to another primarycolor component of light to provide balance between the lightcomponents, the at least one aperture stop including an apodizingaperture stop with an annular color filter corresponding to the at leastone primary color component of light; one or more electronic lightvalves that imparts or impart image information into each of the primarycolor components of light; and first and second sets of relay opticsthat establish three telecentric regions in the system.
 2. The system ofclaim 1 in which the apodizing aperture stop includes a circular annularcolor filter.
 3. The system of claim 1 in which the at least oneapodizing aperture stop includes a cruciform annular color filter. 4.The system of claim 1 further including first and second sets of relayoptics that establish three telecentric regions in the system.
 5. Thesystem of claim 1 in which the one or more electronic light valvesinclude liquid crystal displays.
 6. The system of claim 1 in which theone or more electronic light valves are transmissive liquid crystaldisplays.
 7. The system of claim 1 in which the one or more electroniclight valves are transmissive.
 8. An electronic projection system,comprising: an illumination system generating polychromatic light withplural primary color components: at least one color modifying aperturestop to selectively block at least one of the primary color componentsof light relative to another primary color component of light to providebalance between the light components, the at least one aperture stopincluding an apodizing aperture stop with an annular color filtercorresponding to the at least one primary color component of light; oneor more electronic light valves that imparts or impart image informationinto each of the primary color components of light; and a colorseparation system positioned to receive the polychromatic light providedby the illumination system and to separate the polychromatic light intoprimary color components of light that are directed along separateprimary color component light paths, the at least one color modifyingaperture stop being positioned between the illumination system and thecolor separation system.
 9. The system of claim 8 in which the apodizingaperture stop includes a circular annular color filter.
 10. The systemof claim 8 in which the at least one apodizing aperture stop includes acruciform annular color filter.
 11. The system of claim 8 in which theone or more electronic light valves include liquid crystal displays. 12.The system of claim 8 in which the one or more electronic light valvesinclude transmissive liquid crystal displays.
 13. The system of claim 1in which the one or more electronic light valves are transmissive.
 14. Acolor modifying aperture stop to selectively block a first primary colorcomponent of light relative to a second primary color component of lightcomprising: a non-circular central optical aperture with an elongatedaspect ratio and selected transmissivities of the first and secondprimary color components of light; and an annular color filtercorresponding to the first primary color component of light toselectively block the first primary color component of light relative tothe second primary color component of light.
 15. The aperture stop ofclaim 14 further comprising an opaque outer aperture stop that boundsthe annular color filter.
 16. The aperture stop of claim 15 in which theopaque outer aperture stop forms a circular boundary around the annularcolor filter.
 17. The aperture stop of claim 15 in which the annularcolor filter forms a circular annulus.
 18. The aperture stop of claim 14in which the annular color filter forms a circular annulus.
 19. Theaperture stop of claim 14 in which the central optical aperture includesa substrate.
 20. The aperture stop of claim 14 in which the centraloptical aperture includes a physical aperture.